1. Field of the Invention
The invention relates to the structure of an electric vehicle.
2. Description of Related Art
Vehicles powered partially or entirely by electricity, such as hybrid vehicles that are driven by an engine and an electric motor and electric vehicles that are driven by an electric motor, (hereinafter these types vehicles will simply be referred to as electric vehicles) have come to be widely used. Such vehicles are often configured to use either the front wheels or the rear wheels as driving wheels. However, some of these vehicles are four-wheel drive vehicles that use both the front wheels and the rear wheels as the driving wheels. More specifically, with a four-wheel drive electric vehicle, either the front wheels or the rear wheels are used as the main driving wheels, and the other wheels are used as the auxiliary driving wheels. The main driving wheels are often driven by a synchronous electric motor (hereinafter simply referred to as “synchronous motor”) in which permanent magnets are used in the rotor. On the other hand, the auxiliary driving wheels sometimes serve as driving wheels that drive the vehicle and at other times serve as driven wheels that do not output force to drive the vehicle. Therefore, a configuration is often employed in which, when the auxiliary driving wheels serve as driven wheels, they are driven by an induction electric motor (hereinafter simply referred to as “induction motor”) that does not use permanent magnets in the rotor, in order to suppress the generation of cogging torque caused by whirling of the rotor of the synchronous motor that uses permanent magnets, and an increase in running resistance, that keep the fuel efficiency of the vehicle from being able to improve (see Japanese Patent Application Publication No. 2007-325352 (JP-A-2007-325352), for example).
Further, with an induction motor that does not use permanent magnets, energy loss is large in the low torque output range. Therefore, in an electric vehicle provided with both a synchronous motor that uses permanent magnets in the rotor and an induction motor that does not use permanent magnets, a proposal has been made to increase the energy efficiency of the vehicle by outputting just enough torque from the main driving wheels to compensate for the mechanical loss of the auxiliary driving wheels when the power output to the auxiliary driving wheels is in the low output range (see Japanese Patent Application Publication No. 2008-254677 (JP-A-2008-254677)).
Incidentally, with a synchronous motor that drives the main driving wheels, direct current (DC) power is converted into alternating current (AC) power by an inverter, and a rotor that uses permanent magnets is rotated at a speed synchronized with the rotation speed of a rotating magnetic field generated in a stator by the AC power. Therefore, if the speed of the vehicle is low, the rotation speed of the rotating magnetic field or the frequency of the AC power is low. On the other hand, the torque output from the rotor depends on the amount of current flowing to the electric motor. Therefore, when the frequency of the AC power flowing to the synchronous motor is low and a large amount of current is applied to obtain a large amount of torque, such as when climbing a steep hill at a low speed, the ON/OFF cycle of switching elements of the phases of the inverter may become longer, so the time during which a large amount of current flows to the switching element of a single phase may become longer, and as a result, the temperature of the switching element may rise. In particular, if the vehicle is stopped on a hill and kept in place by the accelerator being depressed (i.e., what will hereinafter be referred to as an accelerator hold state), the frequency of the AC power flowing to the synchronous motor will end up becoming zero, so the switching element of one phase will be continuously on so the temperature of that switching element will rise. However, there is no mention or suggestion of a rise in temperature of the switching element at low vehicle speeds or in an accelerator hold state.